The invention relates to radiation detectors and particularly to an assembly having a radiation detector and a charge transfer readout device.
Such an assembly is used for receiving incident radiation images, particularly infra-red.
The invention relates in particular to assemblies which use solid state detectors, or sensors, which are, for example, photovoltaic diodes or photodiodes; and read out devices which are, for example, charge transfer devices such as charge coupled devices (CCD), connected together by way of an MOS transistor and a transfer gate.
In assemblies of this kind, a certain deterioration of the signal generally occurs. Experience shows that this deterioration occurs principally in the interval between the sensor and the charge transfer device. The cause is faulty injection of the signal from the photodiode into the CCD readout device. It involves the relative values of the impedances of the photodiode and the element into which its signal is injected, namely the connecting transistor between the photodiode and the readout device. This impedance mismatch occurs whatever solid material is used for constructing the diodes, usually lead tin telluride (PbSnTe) or mercury cadmium telluride (HgCdTe) or used for the charge transfer device, generally type n or p silicon.
The difficulty is well known to those skilled in the art. It is clear from the formula of the injection efficiency r which may be written: ##EQU1## and which is nothing more than the distribution of the currents in an assembly of two resistances supplied in parallel with constant current--where R.sub.D is the impedance of the photodiode, and 1/g.sub.m, impedance of the transistor, designating by the term g.sub.m, being its transconductance.
Resistance R.sub.D can hardly be modified. It is fixed in practice by the present state of technology in the field of photodiode construction from the above-mentioned materials.
As for the term g.sub.m, it may be considered as being suitably represented, under the usual conditions of MOS transistors, by ##EQU2## where q, k and T are the charge of the electron, Boltzman's constant, and the absolute temperature; and where I.sub.s represents the total signal current involved in the injection, which current comprises an alternating component i.sub.s corresponding to the variations of the signal during successive readings.
Therefore, to increase the efficiency r--essentially--only I.sub.s is available for a given operating temperature.
Arrangements have already been proposed for increasing this current, which is fixed by the operating point of the diode, by associating therewith an additional current, see: Topics in Applied Physics, vol. 38, p. 78--Charge Coupled Devices--Editor D. F. Barbe Springer Verlag Berlin, Heidelberg, New York.
The object of the invention is to provide a new structure for increasing the current I.sub.s, under the conditions which will be given below.